For many years the second X chromosome in females was viewed as totally inert, due to the process of random X-inactivation. Since it seemed that each sex had only one functional X chromosome, it was assumed that the only genetic difference between sexes derived from Y chromosome-encoded testis-determining genes. Thus, phenotypic differences between the sexes were attributed to differential exposure to gonadal steroids, i.e., estrogen in females and androgen in males. While many sex differences are clearly due to sex steroids, some important distinctions such as the greater longevity of women, are not adequately explained by sex steroid effects. Moreover, it is difficult to reconcile the view that only one X chromosome was actually functional in normal females with the fact that there is a distinct phenotype in girls with monosomy for the X chromosome, or Turner syndrome (TS). Indeed, the study of TS shows that the 2nd X chromosome is essential for normal female development.[unreadable] [unreadable] X-chromosome and congenital cardiovascular disease: We now know that 15-20% of genes on the 2nd X chromosome escape X-inactivation. Pseudoautosomal X-chromosome genes have Y chromosome homologues and behave like autosomal genes with expression from both X chromosomes in females and from X- and Y-chromosomes in males. For example, haplo-insufficiency for a pseudoautosomal gene known as SHOX causes short stature and skeletal defects in TS. We hypothesize that the congenital cardiovascular defects (CHD) found in about 50% of patients with TS are due to haploinsufficiency for an as yet unknown pseudoautosomal gene. Supporting this view, we have mapped the critical region for CHD in TS patients to the distal short arm (Xp; Matura et al., 2007; Sachdev et al., 2008), which contains the major pseudoautosomal region. Moreover, the murine X chromosome has lost the Xp pseudoautosomal region, and the mouse has a minimal XO phenotype; in contrast, the cat and dog, which have high Xp pseudoautosomal homology to the human, have an XO phenotype similar to human patients, including CHD. Thus we are focusing on candidate Xp pseudoautosomal genes that are conserved in canine, feline and human. TS is the leading cause of fatal aortic dissection in young women. Previously it was unknown if all women with TS were at risk, or if only those with known CHD were at risk of acute aortic dissection. Moreover, it was unknown if aortic root dilation precedes and predicts aortic degeneration as in Marfan syndrome. We recently reported the first prospective measure of the incidence of aortic dissection in TS and proposed new guidelines to identify high risk patients (Matura et al., 2007). We showed that aortic dilation at the level of the ascending aorta (similar to pattern in non-syndromic bicuspid aortic valve) but not particularly at the aortic root (as in Marfan syndrome) was indeed predictive of aortic dissection, but dilation was only recognized after normalization for the small body size of TS women. [unreadable] [unreadable] X-chromosome, genomic imprinting and longevity: Genomic imprinting of X-chromosome gene expression may also to male-female differences and to the TS phenotype. Genomic imprinting involves the selective expression of certain genes determined by their parental origin; imprinting of X-linked genes causes different gene expression in males and females, since normal women are mosaic for maternally and paternally inherited active X chromosomes (XM and XP), while men are monosomic for XM. Genes imprinted (silenced) on XM would still be expressed in females from 50% of cells, but not expressed in males. Women enjoy greater longevity than men mainly due to their lower risk, across all age groups, for ischemic heart disease. The key advantage women have in this regard is their salutary, gynoid fat distribution, meaning adipose tissue preferentially concentrated subcutaneously in the hips and thighs. In contrast, men tend to concentrate fat in the abdominal area, especially intraperitoneal visceral fat, which has many adverse metabolic effects, including an atherogenic lipid profile and a chronic inflammatory state associated with high cardiovascular risk. Estrogens seem to promote t he peripheral disposition of apparently healthy fat in maturing females, but testosterone is not responsible for the selective accumulation of abdominal fat in males. Based on observations on the body habitus of women with TS and a sole XM, we hypothesized that parental imprinting of X-linked genes involved in regional fat distribution and lipid metabolism may promote android, or central fat accumulation. To test the hypothesis we compared regional fat distribution and lipid profiles in groups of women that were monosomic for XM vs. XP. The XM and XP groups had similar BMI and total body fat, but women with a single maternally inherited X-chromosome had 2-fold greater abdominal and specifically intra-abdominal, or visceral fat compared to those with XP. This was associated with a distinctly atherogenic lipid profile compared to the paternal X group. The male-type fat distribution and lipid profile in XM women supports the view that differential X-chromosome gene dosage, determined by genomic imprinting, contributes to abdominal adiposity and excess ischemic heart disease in 46,XY men. [unreadable] Because of the different parental origins of the X chromosome in males and females, in contrast to autosomal imprinting, X genomic imprinting should be associated with differential effects depending on the sex of the offspring as well as the parent of origin. Thus, we predicted that X-linked imprinting does not regulate traits that are not sexually dimorphic such as renal or cardiac development. We proved this by showing equal prevalence of renal and cardiovascular defects in XM vs. XP groups with TS. Somatic size, however, is sexually dimorphic (men >> women), and we have provided evidence that this is related to maternal X chromosome (Bondy et al, 2007).[unreadable] [unreadable] Growth hormone (GH) treatment in TS: We evaluated the effects of GH treatment on body composition and glucose tolerance in girls with TS (Wooten et al., 2008). Seventy five percent of the girls had been or were currently treated with GH. The average age of starting GH was 8 yrs and average treatment duration was 4 years. A most remarkable finding was that glucose tolerance was actually better in the GH-treated group, an effect that was attributable to the more salutary body composition of the treated girls. The average BMI, total body fat (DXA) and abdominal fat (MRI) was significantly greater in GH naive girls vs girls currently on GH and those that had finished GH treatment. These unexpected findings were quite striking and the statistical significance was very robust even withstanding a rigorous correction for multiple comparisons. The excessive abdominal adiposity in untreated girls was associated with reduced insulin sensitivity and impaired glucose tolerance. Thus girls treated with GH during childhood seemed avoid the development of central, abdominal adiposity and the adverse metabolic phenotype typical of many girls with TS. The present findings are novel and remarkable because it was predicted by some that GH treatment would increase insulin resistance and risk for diabetes in girls with TS. To the contrary, this study suggests that untreated girls may be at greater risk for diabetes due to their excessive adiposity.